Some proposals have already been made regarding the use of a reticulated metal sheet, having open cells, for the production of battery electrodes. For example, U.S. Pat. No. 3,694,325 proposes a method of production of fairly uniform nickel sheet (uniformity across the direction of the thickness) having a comparatively smaller ratio of plated nickel metal between cell strands at the surface area of the sheet and cell strands at the central part of the sheet, in the direction of thickness of the sheet. As discussed in the patent, there can usually be a ratio of the outermost fiber plating deposit thickness, to the innermost fiber plating deposit thickness. This ratio can be termed a deposit thickness ratio, also called a differential thickness ratio or, in either case, the DTR. The U.S. Pat. No. 3,694,325 proposes a batch process of sheet preparation and a uniform reticulated nickel sheet which might have a DTR as small as 1.05:1 (plated Ni thickness at the surface area: thickness at the central part). The patent suggests that such a material can be used as an electrode of a fuel cell.
In addition, Japanese Public Disclosure No. 26435/1977 and Japanese Patent Publication No. 54235/1987 (U.S. Pat. No. 4,251,603) propose some battery electrode structures by impregnating active material into the void area of such reticulated nickel sheet.
In the production process of making reticulated metal sheet by an electrical plating method, in general, such sheet is produced using sponge-like plastic foam having open cells, as starting material. The plastic foam is pretreated to have threshold electrical conductivity such as by some electroless plating or painting procedure, then the electrically conductive foam is electrically plated by installing such foam as a cathode. The anode(s) is placed either at one surface or both sides of the foam sheet. In order to obtain a reticulated metal sheet having a uniform plating thickness into the direction of thickness, it is common to set an anode at both sides of the plastic foam and to conduct the plating on both sides at one time or on each side, one side at a time. In such a process, electric currents at the cathode have a tendency to gather at the surface of the foam sheet. Therefore it can become difficult to flow such currents into the central part of the sheet. Though various methods have been developed to get good current flow, if the thickness of a foam becomes thicker, or if the diameter of pores of a foam become smaller, it becomes more difficult to obtain a reticulated metal sheet having a small DTR. Particularly, it is substantially very difficult, according to the conventional plating method, to produce reticulated metal sheet having a DTR of 1:1.
Accordingly, U.S. Pat. No. 3,694,325 states that reticulated metal foam having DTR's up to 2.0:1 can be considered to be substantially uniform. The patent also suggests an electrode of a fuel cell as a possible application of such reticulated nickel sheet having larger DTR. Electrodes of fuel cells generally comprise a catalyst or the like by depositing on the surface of the internal void part of the sheet. A fuel gas passes through the void part to carry out a reaction on the surface of the catalyst. Therefore a reticulated material having a large internal space because of a large DTR will be desirable.
Japanese Public Disclosure No. 26435/1977 proposes a sandwich like electrode structure having two kinds of reticulated nickel sheets each of different porosity. A nickel sheet with larger porosity is placed at the inside as a core material and both sides are covered with smaller porosity sheet, to reduce the dropout of the impregnated active material from the internal sheet.
Japanese Patent Publication No. 54235/1987 contains test data which shows that the rate of loss of the active materials impregnated into the metal foam sheet as paste, can be improved by using the metal foam sheet having comparatively smaller pore diameter at both surfaces as contrasted to the inside of the sheet. Such pore size difference is naturally formed when the metal foam sheet is produced by electrical plating method. The use of reticulated metal sheets having larger DTR as battery electrodes has been proposed. In the Japanese Patent Publication No. 54235/1987, there is data showing the increasing filling density with increasing DTR, (or pore diameter). In addition, there are two examples with reticulated nickel foam sheet produced with the DTR substantially close to 1:1 and with a high DTR. Then, it is reported that the loss of the impregnated active material decreases with the increase of DTR of the sheet. By conducting the repeating electrical charge--discharge test (cycle test), it is concluded that a battery life is extended by using sheets with large DTR.
A reticulated metal sheet having a large DTR is generally inferior to a small DTR in physical properties; particularly in elongation, compression and bending properties. In the battery electrode application, the metal sheet is generally wound into cylindrical form. Then a sheet having poor physical properties will cause problems in the process of battery production. Other problems occur when using foam sheet with poor physical properties, such as battery performance is not uniform, battery-to-battery, and also the produced batteries may not achieve the design performance.
One reason is nonuniformity in various parts of the sheet, but another reason is large DTR. The strand strength near the central part of the sheet is not as strong due to the small amount of plated metal; thus there will be more breakage of the strands in the center area than in the external surface areas when the sheet is fabricated into a battery. Current collecting ability and electric conductivity will differ between the inside and outside of the sheet and thus the electrode cannot perform at design specifications.
A method has been disclosed of producing a reticulated metal strip continuously in U.S. Pat. No. 4,978,431. In addition, there is now disclosed a reticulated metal foam strip with small DTRs, which can be achieved by continuously and economically improving the process of the patent.
Following this invention, further research was conducted to develop an electrode having not only a large filling density of active material but also a superior overall performance as a battery, using a reticulated metal sheet with a DTR of 1.1-3.5:1 produced by utilizing this invention. Consequently a high performance electrode and its manufacturing procedure were produced based on the new findings.